Acoustic apparatus



Jan. 21, 1964 s. wALczAK AcousTIc APPARATUS Filed nec; 29, 1961 AJA INVENTOR. je Vif/f@ H4/cur BY A ifm/fray United States Patent O 3,118,972 ACOUTIC APPARATUS Sylvester Walczak, Indianapolis, Ind., assigner to Radio Corporation of America, a corporation of Delaware Filed Dee. 29, 1961, Ser. No. 163,170 Claims. (Cl. 179-1) This invention relates to acoustic apparatus, and more particularly to improved acoustic transducers.

The invention is especially suitable for use in loudspeakers and provides loudspeaker apparatus which can be compensated for distortion and in which the fidelity of sound reproduction is improved.

Distortion in a loudspeaker may be attributable to nonlinearities in the mechanical suspension system of the loudspeaker and inhomogeneity of the magnetic flux in the voice coil air gap. Because of this distortion, the signals developed in the electrical driving system for the loudspeaker and the amplitude of the motion of the cone of the loudspeaker do not correspond linearly to each other.

It has been proposed to reduce distortion in loudspeakers by using a feedback system in which a portion of the output of a loudspeaker is returned to the input of the electrical driving system of the loudspeaker. Such feedback systems have been difficult to use since, unless special precautions are taken, oscillations (sometimes in the form of howling) occur in the loudspeaker system.

Accordingly, it is an object of the present invention to provide improved acoustic apparatus which may be compensated :for distortion.

it is a further object of the present invention to provide improved loudspeaker apparatus adapted for use with a distortion compensation feedback system, which apparatus provides for oscillation free operation.

It is a still further object of the present invention to provide improved pickup means in a loudspeaker apparatus for deriving a signal corresponding to the output of the loudspeaker, which signal is useful in a feedback distortion compensation system for the loudspeaker.

lt is a still further object of the present invention to provide an improved device for picking up signals corresponding to the output of a loudspeaker, which device is low in cost and simple to install on the loudspeaker.

It has been found that the leakage or fringe flux from a magnetic structure of a moving coil electromagnetic transducer, such as a loudspeaker, corresponds to the motion of the coil. The leakage flux from a loudspeaker magnetic structure thus corresponds to the loudspeaker acoustic output.

An acoustic transducer embodying the invention includes a magnetic structure. Another magnetic structure carrying a pickup coil is attached to the magnetic structure of the transducer so that leakage flux from the transducer magnet structure threads the pickup coil. The signal derived by the pickup coil corresponds to the motion of the moving coil of the transducer and to the output of a loudspeaker type transducer. This signal is thus useful in providing distortion compensation and fidelity i iprovement by feedback.

The invention itself, both as to its organization and method of operation, as Well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings in which:

FIG. l is a front View, partially in section, of loudspeaker apparatus of one type embodying the present invention;

FIG. 2 is a fragmentary front view, partially in section, of another type of loudspeaker apparatus embodying the invention; and

3,118,972 Patented Jan. 21, 1964 ICC FIG. 3 is a circuit diagram of a loudspeaker system embodying the invention.

Referring more particularly to FIG. l, there is shown a loudspeaker 8 having a cone-shaped diaphragm 10 connected at one end to a cylindrical voice coil structure 12 on which a Voice coil 14 is Wound. The other end of the diaphragm 1l) is connected at its periphery to a frame 16, usually called a dishpan.

A magnetic field structure 18 supports the dishpan 16. The field structure 18 includes a U-shaped (in crosssection) yoke 2t? of magnetic material, such as cold rolled steel, which is closed by a yoke plate 22, also of magnetic material. The yoke plate 22 has a circular opening 2d therein. A pole member, including a cylindrical magnet 2d and a pole piece 28, is centrally disposed in the yoke 20. The magnet 26 may be of permanent magnet material, such as Alnico, which is an alloy of aluminum, nickel, cobalt, copper and iron. The pole piece 2S is also centrally disposed with respect to the opening 24 in the yoke plate 22 by means of a centering and clamping ring 3u of non-magnetic material, such as brass. The ring 30 also holds the magnet and pole piece in assembled relation in the yoke 20. The pole piece 28 is cylindrical in shape and defines a cylindrical voice coil air gap 29 between the peripheral surface of the yoke plate opening 24 and the pole piece outer peripheral surface. The voice coil 12 is centered within the voice coil air gap by means of a resilient centering disk 32, usually called a spider. The loudspeaker described above is substantially similar in construction to a loudspeaker described in Cunningharn Patent 2,501,031, issued March 2, 1950.

A device 34 for picking up a signal corresponding to the sound output of the loudspeaker is attached to the exterior of the magnetic structure 18. This device 34 includes a second magnetic structure having a U-shaped (in cross-section) yoke 36 of magnetic material, such as cold rolled steel, similar in shape to the loudspeaker yoke 2d but smaller in size. Centrally disposed within this yoke is a cylindrical pole member 38, also of magnetic material. This cylindrical pole member 38 may be secured to the yoke 3d by means of soldering,'ce inenting, or with a screw (not shown) extending through the bottom of the yoke 34 into the cylindrical pole member 3S. The top of the cylindrical pole member 3S has a boss 4t! of reduced diameter thereon. This boss is received into a hole 41 in the bottom of the yoke 15 of the loudspeaker. The magnet structure 36--38 may be held on the loudspeaker yoke 18 by securing the boss 4d in the hole 41 by means of a force fit. Alternatively, the boss tu may be swaged onto the loudspeaker yoke 29 before the loudspeaker magnet structure, including the magnet 26, the pole piece 28, the yoke plate 22 and the centering ring 3), is assembled. The hole 41 in the bottom of the loudspeaker yoke 20 and the magnet 26 are desirably coaxial with each other. Thus, the pole member 38 is disposed coaxially with respect to the magnet 26. Other techniques, such as cementing, soldering, or swaging the yoke 36 to the bottom of the speaker yoke 20, may be used to secure the pickup device to the loudspeaker. A pickup coil 42 is wound around the pole member 38 and serves to derive an output signal voltage.

In operation, the m-agnet 26 establishes magnetic flux along flux paths shown by the dashed line. These flux paths extend through the magnet 26, the pole piece 28, across the voice coil air gap and back to the magnet by Way of the yoke plate 22 and yoke 2i?. It will be appreciated that the flux passes through the voice coil air gap 29 and the pole plate 22 in a radial direction. It is ordinarily the practice to use a magnet 26 which provides a magnetic potential sufficient to establish a flux density in the yoke 20 and yoke plate 22 which is close to saturation. Thus, a signicant amount of fringe flux emanates from the yoke 20 and the pole plate 22. This fringe flux is intercepted by the yoke 36 of the pickup device 34 which directs the fringe ux along paths through the yoke 36 and pole member 38 of the structure. This fiux threads the coil 42.

When the loudspeaker is operated by its electrical driving system, such as the output of an amplifier, an alternating current is established in the voice coil 14. This alternating current establishes an alternating magnetic flux which interacts with the direct magnetic flux established by the magnet 26 and effectively modulates this direct magnetic flux. The modulation of the flux is related to the motion of the voice coil `14 since the inliuence of the magnetic field set up by the coil 14 on the direct magnetic flux depends upon the position of the voice coil in its air gap. In other words, the coupling of magnetic flux from the voice coil into the magnetic structure 18 of the loudspeaker depends upon the position and the motion of the voice coil. When the amplitude of the diaphragm is large, the excursions of the voice coil 14 will be greater relative to the voice coil gap than when the amplitude of the cone 10 is small. The modulation of the magnetic flux in the magnetic structure 18 of the loudspeaker is, therefore, a function of the motion of the voice coil 14.

Since the motion of the voice coil depends upon the nonlinearities of the suspension system for the diaphragm and voice coil, and also depends upon the inhomogeneity of the ux density in the air gap, the modulated flux carries information as to distortion in the loudspeaker. The fringe flux which is picked up by the pickup device 34 also contains modulation components due to the motion of the voice coil. Accordingly, the output of the pickup coil 42 is related to the motion of the voice coil 14. The acoustic output of the loudspeaker is related to the motion of the voice coil. Hence, the signal derived by the pickup coil 42 is related to the output of the loudspeaker. The driving signal applied to the voice coil will also be inductively coupled to the coil 42 in the pickup device 34. However, the voltage output of the pickup coil 42 carries information as to the distortion of the loudspeaker output which may be utilized in a feedback system for compensating such distortion.

Referring to FIG. 2, there is shown a portion of a cupshaped loudspeaker magnetic field structure of the type described in Hamson Patent 2,964,597, issued December 13, 1960. This field structure includes a cylindrical, cupshaped, yoke structure 50. A cylindrical magnet 52 is disposed centrally within the yoke structure 50 and is held in place therein by a deformed retaining ring 54. A frame S6 supports the diaphragm 58 of the loudspeaker and is mounted on the open end of the yoke Si). This frame carries a resilient centering disk or spider 60 which supports the truncated yapical end of the cone. A voice coil form 62 is secured to the apical end of the cone and carries a voice coil 64. The voice coil 64 is essentially disposed in an air gap defined between the outer periphery of the magnet 52 and the inner periphery of the cup-shaped yoke 50. The bottom of the yoke 50 has an opening 66 therein.

A pickup device 68 is secured to the bottom of the yoke 50. 'Ihis pickup device 68 includes a cup-shaped yoke 70 of magnetic material having a center pole member 72. This center pole can be formed by machining a cylindrical slot in the cup-shaped yoke 70. A boss 74 is formed on the top of pole 72. This boss 74 may be secured in the hole 66 by a force fit, by swaging, or by other techniques, as described in connection with FIG. 1. The device 68 and the yoke 5i) may be assembled readily by attachment of the boss 74 in the hole 66. A coil 78 is wound around the pole 72. Flux established by the magnet 52 in the yoke 50 extends radially across the voice coil4 air gap and then downwardly through` the yoke, re`

turning by way of the bottom of the yoke to the magnet. Although the diameters of the loudspeaker yoke structure 50 and the pickup yoke 70 are shown to be about equal to each other, the diameter of the pickup yoke may be smaller. Preferably, the diameter of the pickup yoke is larger than the diameter of the magnet 52. A substantial amount of fringe flux emanates from the yoke 50, especially when the magnet has sufiicient magnetic potential to establish a ilux density in the yoke 50 close to magnetic saturation, `as is generally the practice. This fringe ux extends downwardly into the yoke 76, radially across the bottom of the yoke, and then upwardly through the pole member 72 into the magnet 52. The coil 78 derives an output signal voltage which corresponds to the motion of the coil 64 and to the output of the loudspeaker, in the same manner as does the coil 42 (FIG. l).

The disposition of the pickup devices at the bottom of the yoke 18 (FIG. 1) or the yoke 50 (FIG. 2) increases the fringe flux which threads the respective coils 42 and 78 because the yokes 36 and 70 of the pickup devices shape and direct the fringe field through the pickup coils 42 or 73. The pickup coils 42 of FIG. 1 or 78 of FIG. 2 are also protected from damage, since they are surrounded by the yoke structures 36 and 70 of their respective pickup devices 34 and 68. Since the flux turns sharply from the yoke into the magnet at the bottom of `the yoke structures, the greatest fringing of flux occurs near the bottom of the yoke. Thus, the pickup devices 42 and 78 are disposed in close proximity to the area of maximum leakage tiuX from the magnetic structures of their respective loudspeakers.

The magnetic coupling of audio signals applied to the voice coils I14 and 64 between these voice coils and their associated pickup coils 42 and 78 is relatively small, since the voice coils and pickup coils are substantially magnetically isolated from each other by the back part of the yoke structure 18 and 59. It will be observed that this isolation does not prevent the effective modulation due to the motion of the voice coil of the fringe flux to which the pickup coils 42 and 78 are responsive. Such isolation of the pickup coils and voice coils is believed to be responsible for the stability of feedback systems employing the pickup devices 34 and 68. Feedback systems including these devices 34 and 68 are not very susceptible to oscillation.

A loudspeaker system employing feedback from such a speaker as that illustrated in FIG. l or 2 is shown in FIG. 3. This loudspeaker system includes the audio stages of a transistor radio. The illustrated audio stages alone, without the feedback feature are used in RCA Victor TX-l series transistor radios (chassis No. RC- 1196A), as described in a publication of Commercial Service, RCA Service Company, Camden 8, New Jersey, entitled, Service Data 1959 No. 20 (first edition, first printing, June 29, 1959).

A loudspeaker 80 which may be similar to either one of the loudspeakers shown in FIG. 1 or FIG. 2, but having a center tapped voice coil 81, is equipped with a pickup device 82 similar to the pickup devices 34 or 63 of the loudspeakers shown in FIG. 1 or FIG. 2. The electrical driving system for the loudspeaker receives an input signal, for example, from the detector of the radio receiver. This signal is coupled through a volume control 84 to a driver yamplifier stage 86. The `driver amplifier drives a push-pull amplifier 88. Output signals amplified by the amplifier 88 are applied directly across voice coil 81 of the loudspeaker 80. The voice coil yis tapped, and no output transformer Lis used. The pickup device 82, therefore, advantageously provides a feedback voltage even though an output transformer is not used. The pickup device is connected between A.C. ground 91 and the near ground end 93 of the volume control 84. The output signal from the device 82 is Itherefore applied through the control 84 to the base of the transistor in the driver stage 86. A resistor is connected across the device 82 to attenuate the pickup signals somewhat. A capacitor 92 is also connected across lthe device 82 to limit the high frequency components of the output signal from the device 82. These components 901 and 92 may or may not be needed, depending upon the type oef loudspeaker and the number ci turns in the coil of the device 82. The connections to me device `are made so that the output signals from the device 82 are phased oppositely With respect to the audio signals applied to the amplifier stage 76 from the detector. In other Words, the pickup ooil 72 provides a feedback system which couples the output of the loudspeaker and the input of its driving system in negative feedback relationship. The loudspeaker system including the feedback system is compensated for distortion due to nonlinearities in the motion of the loudspeaker voice coil.

From the foregoing description, it Will 'be apparent that there has been provided improved acoustic apparatus, especially adapted for use in loudspeaker systems. The apparatus provides for the reduction of distortion due to nonlinearities in the operation of the loudspeaker in a convenient and economical manner. The invention also provides for distortion compensating and frequency response improving feedback in loudspeaker systems without the 'danger of introducing oscillations in the system. While two embodiments olf the loudspeaker apparatus embodying the invention, and one embodiment of the electrical system including a loudspeaker embodying the invention have been illustrated, it will be appreciated that variations in the illustrated apparatus and system, Within the scope of the invention, Will undoubtedly suggest themselves to those skilled in the art. Accordinvly, the foregoing description shoulld be Itaken as illustrative and not in any limiting sense.

What is claimed is:

1. ln an acoustic transducer having a magnetic field structure inclu-ding `means for establishing magnetic ux in said structure, said stnucture also having an opening therein and having a coil movable in said opening, the improvement comprising another magnetic structure mounted on the outside of said first-named structure and exposed to portions off said magnetic iiux which fringe from said first-named structure, and a pickup coil Wound on said other structure for deriving an output voltage in response to changes in said fringe fiuX.

2. In an acoustic transducer having a magnetic field structure including a yoke structure and a pole member defining -a voice coil air gap therebetween, said pole member including a magnet, said transducer also having a voice coil movable in said air gap and a diaphragm movable with said voice coil, the improvement comprising a pickup device comprising another magnetic field structure having a yoke structure and a pole member within said yoke, and a pickup coil Wound around said lastnamed pole member, said ilast-named yoke structure being mounted on said transducer yoke structure 'with said lastnamed pole member in alignment with said transducer pole member.

3. ln loudspeaker apparatus having a magnetic field structure including a U-shapcd yoke, a yoke plate on said yoke having an opening therein, a cylindrical pole member witnin said yoke centrally ydisposed with respect to said opening for defining with the periphery of said yoke plate opening a voice coil air gap, said pole member including a magnet, the improvement comprising a pickup device, a magnetic held structure including a U-shaped yoke smaller Athan said loudspeaker yoke, and a cylindrical pole member in said yoke ,and a pickup coil wound around said pole member, said pickup device field structure being mounted on the outside of sai-d loudspeaker field structure with the free ends of said device yoke and pole member in Contact with the outer surface of said loudspeaker yoke, said device pole member being disposed coaxially with said loudspeaker pole member.

4. in loudspeaker apparatus including a magnetic lield structure having a cylindrical, cup-shaped yoke, and a cylindrical magnet centrally disposed in said yoke and attached thereto, the improvement comprising a pickup device including a cylindrical cup-shaped yoke having a central cylindrical pole member, said last-named yoke and pole member being mounted ion the outside of said loudspeaker yoke on the closed end theneoi with said device poie member coaxial with said magnet, and a pickup coil Iaround said device pole member.

5. A loudspeaker system comprising a loudspeaker having a magnetic field structure including a yoke of magnetic material, a pickup device including a pickup coil mounted on said yoke md responsive to fringe magnetic flux emanatingy fro-m said yoke, an electrical system responsive to electrical audio signals `for `driving said loudspeaker to reproduce sound corresponding tio said signals, and means for connecting said pickup coil and said electrical driving system in feedback relationship whereby to compensate for distontion in said loudspeaker.

References Cited in the lile of this patent UNITED STATES PATENTS 1,734,944 Green Nov. 5, 1929 1,901,331 Pridham Mar. 14, 1933 2,860,183 Conrad Nov. 1l, 1958 2,964,597 Harrison Dec. 13, 1960 3,047,661 De Winkcr July 31, 1962 3,057,961 Turner Oct. 9, 1962 3,061,675 Macdonald Get. 30, 1962 

5. A LOUDSPEAKER SYSTEM COMPRISING A LOUDSPEAKER HAVING A MAGNETIC FIELD STRUCTURE INCLUDING A YOKE OF MAGNETIC MATERIAL, A PICKUP DEVICE INCLUDING A PICKUP COIL MOUNTED ON SAID YOKE AND RESPONSIVE TO FRINGE MAGNETIC FLUX EMANATING FROM SAID YOKE, AN ELECTRICAL SYSTEM RESPONSIVE TO ELECTRICAL AUDIO SIGNALS FOR DRIVING SAID LOUDSPEAKER TO REPRODUCE SOUND CORRESPONDING TO SAID SIGNALS, AND MEANS FOR CONNECTING SAID PICKUP COIL AND SAID ELECTRICAL DRIVING SYSTEM IN FEEDBACK RELATIONSHIP WHEREBY TO COMPENSATE FOR DISTORTION IN SAID LOUDSPEAKER. 